We are interested in learning more about the factors contributing to adolescent substance use and other potentially harmful behaviors. Identifying these factors will provide policymakers and parents with better information, and will hopefully lead to effective strategies for reducing the incidence of teen substance use, early sexual activity, and criminal behavior. [unreadable] [unreadable] According to what have been labeled "epidemic models" of behavior, children are directly influenced by the aspirations and actions of their peers. Individuals growing up in a neighborhood or attending a school in which a particular behavior is prevalent are themselves more likely to engage in that behavior. Although the notion of contagious behavior has an intuitive appeal, attempts to document the precise role played by peers in the determination of child outcomes have not been entirely successful. For instance, researchers working in this area have typically assumed that peer characteristics and behavior can be treated as exogenous. However, if parents select their neighborhood (or the school to which they send their children) in part based on the characteristics of other children in the neighborhood (or other students in the school), then this assumption can lead to biased estimates of peer group effects. [unreadable] [unreadable] Our research avoids this obstacle by taking advantage of the fact that many risky behaviors (including tobacco and marijuana use, drinking, and sexual activity) are much more prevalent among older teens than among younger teens. The positive relationship between age and these behaviors suggests that children will be exposed to different environments depending on the relative age of their peers. Using data from the National Longitudinal Survey of Youth 1997 cohort, and the National Education Longitudinal Study of 1988, we examine the impact of having older classmates and siblings on a variety of risky adolescent behaviors. Additional analyses examine positive behaviors. [unreadable] [unreadable] [unreadable]